The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPPthird generation partnership projectBMCbroadcast/multicast controlDLdownlink (network towards UE)E-DCHenhanced dedicated (physical) channelE-HICHE-DCH HARQ acknowledgement indicator channelE-RGCHE-DCH relative grant channelE-RNTIE-DCH radio network temporary identifierFACHforward access channelHARQHybrid Automatic Repeat RequestH-RNTIHS-DSCH radio network temporary identifierHSDPAhigh speed downlink packet accessHS-DSCHhigh speed downlink shared channelHSPAhigh speed packet accessMBMSmultimedia broadcast/multicast serviceNCLneighbor cell listNode Bbase stationPLMNpublic land mobile networkRFradio frequencyRNCradio network controllerRRCradio resource controlSIsystem informationUCIDunique cell identifierUEuser equipmentULuplink (UE towards network)UTRANuniversal terrestrial radio access networkWCDMAWideband Code Division Multiple Access
Continuing improvements of the UTRAN system have recently included the investigation of a CELL-FACH enhancement which in part intends to improve cell reselection. A UE in the CELL-FACH state has no dedicated physical channel allocated to it, but instead it continuously monitors the FACH in the DL and is assigned a random access channel RACH for accessing an uplink transport channel. While in the CELL-FACH state the UE can have only one serving cell, and so it performs cell reselection according to specified reselection rules, typically to change the current serving cell to a better quality neighbor cell.
In current specifications the UE does this by first obtaining the SI of the neighbor cell (which takes typically 1.28 seconds) then sends a cell update message upon cell reselection while in the CELL-FACH state. The network then provides in a Cell Update Confirm message a dedicated resource on the new serving cell for the UE. The cell update procedure can take up to a full second to perform, and so until it is complete the UE cannot perform user data transmission and reception. This means that there is a time, on the order of up to a few seconds, during which service is disrupted due to the cell reselection.
The UEs utilize the various cells' SI for many purposes, including determining if the UE is allowed to attempt reselection to that cell and the configuration the UE should use to do so. This means that the UE must have the SI of the cell to which it attempts to reselect even before beginning the cell reselection process. This fact gives rise to two issues for obtaining the neighbor cell's SI, which the cell transmits on its broadcast channel. First, it may take several seconds for the UE to obtain the SI meaning access to the cell can be delayed. Second, there are certain UE connection states, such as the CELL-FACH state noted above, during which the UE cannot read the neighbor cell's SI (or at least the UE's reading of it is limited).
The UE is allowed to cache SI. In this regard, the UTRAN system provides at 3GPP TS 25.331 v8.1.1.3:                The UE may store system information blocks with cell, PLMN or Equivalent PLMN area scope (including their value tag if applicable) for different cells and different PLMNs, to be used if the UE returns to these cells.        
But any given UE over time transits through quite a large number of cells, so storing every cell's SI quickly becomes an issue of memory management. Additionally, while any given cell's SI typically remains static, SI is in fact semi-dynamic and so the SI which a UE stores in the morning may or may not be valid for that same cell when the UE passes through it again in the afternoon.